Effects of microstructure on high-cycle fatigue properties of Ti-4Al-6Mo-2V-5Cr-2Zr alloy

To clarify effects of microstructure on fatigue crack initiation mechanism of Ti-4Al-6Mo-2V-5Cr-2Zr alloy, highcycle fatigue behavior of hierarchical nanostructure (HN) and lamellar microstructure (LM) was investigated. Fatigue tests results show that the high-cycle fatigue property of LM (50.1(107), 685 MPa) is better than that of HN (50.1(107), 630 MPa). EBSD analysis and TEM observation demonstrate that with cyclic loading of stress, prism < a > slip is activated in alpha p phase, and basal < a > slip is activated in grain boundary alpha phase (GB alpha). Plastic deformation of HN mainly occurs in short rod alpha(p) phase and GB alpha. Fatigue cracks preferentially initiate at weak parts of these structures and propagate into a transformed beta structure (beta trans). In addition, the intrinsic reason for the random behavior of the fatigue life can be attributed to differences in the microstructure ahead of the main crack tips. Fatigue data for HN containing GB alpha and short rod alpha p shows high scattering. Dislocation slip in LM occurred mainly in beta matrix and piled up at alpha s/beta interfaces and grain boundaries (GBs). Local high stresses caused {0111} nano-scale twins in alpha s phase to harmonize local plastic deformation, which makes cracks more preferable to nucleate at beta GBs.